The present disclosure generally relates to ion implantation systems, and more particularly, to a reduced trace metal contamination ion source of the type typically used in ion implantation systems, which exhibits improved corrosion resistance within the arc chamber of the ion source.
Ion implantation systems or ion implanters are widely used to dope semiconductors with impurities in integrated circuit manufacturing, as well as in the manufacture of flat panel displays. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam of desired energy. The ion beam is then directed at the surface of the workpiece, such as a semiconductor wafer, in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. The implantation process is typically performed in a high vacuum process chamber, which prevents dispersion of the ion beam by collisions with residual gas molecules and minimizes the risk of contamination of the workpiece by airborne particles. A typical ion implanter includes an ion source for generating the ion beam, a beamline including a mass analysis magnet for mass resolving the ion beam, and a target chamber containing the semiconductor wafer or other substrate or workpiece to be implanted by the ion beam, although flat panel display implanters typically do not include a mass analysis apparatus. For high energy implantation systems, an acceleration apparatus may be provided between the mass analysis magnet and the target chamber for accelerating the ions to high energies.
Conventional ion sources include a plasma confinement chamber having an inlet aperture for introducing a gas to be ionized into plasma and an exit aperture opening through which the plasma is extracted to form the ion beam. One example of a dopant gas is phosphine. When phosphine is exposed to an energy source, such as energetic electrons or radio frequency (RF) energy, the phosphine can disassociate to form positively charged phosphorous (P+) ions for doping the workpiece, as well as disassociated hydrogen ions. Typically, phosphine is introduced into the plasma confinement chamber and then exposed to the electron source to produce both phosphorous ions and hydrogen ions. The plasma comprises ions desirable for implantation into a workpiece, as well as undesirable ions which are a by-product of the dissociation and ionization processes. The phosphorous ions and the hydrogen ions are then extracted through the exit opening into the ion beam using an extraction apparatus including energized extraction electrodes. Examples of other typical dopant elements of which the source gas is comprised include phosphorous (P), arsenic (As), or Boron (B), and many others.
The dosage and energy of the implanted ions are varied according to the implantation desired for a given application. Ion dosage controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dosage applications. Ion energy is used to control junction depth in semiconductor devices, where the energy levels of the ions in the beam determine the degree of depth of the implanted ions. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. In addition, the continuing trend toward higher device complexity requires careful control over the uniformity of implantation beams being scanned across the workpiece.
The ionization process in the ion source is achieved by excitation of electrons, which then collide with ionizable materials within the ion source chamber. This excitation has previously been accomplished using heated cathodes or RF excitation antennas. A cathode is heated so as to emit electrons which are then accelerated to sufficient energy for the ionization process, whereas an RF antenna generates electric fields that accelerate plasma electrons to sufficient energy for sustaining the ionization process. The antenna may be exposed within the plasma confinement chamber of the ion source, or may be located outside of the plasma chamber, separated by a dielectric window. The antenna is energized by an RF alternating current which induces a time varying magnetic field within the plasma confinement chamber. This magnetic field in turn induces an electric field in a region occupied by naturally occurring free electrons within the source chamber. These free electrons accelerate due to the induced electric field and collide with ionizable materials within the ion source chamber, resulting in plasma currents within the ion chamber, which are generally parallel to and opposite in direction to the electric currents in the antenna. Ions can then be extracted from the plasma chamber by one or more energizable extraction electrodes located proximate a small exit opening, so as to provide a small cross-section (relative to the size of the workpiece) ion beam.
Batch ion implanters include a spinning disk support for moving multiple silicon wafers through the ion beam. The ion beam impacts the wafer surface as the support rotates the wafers through the ion beam.
Serial implanters treat one wafer at a time. The wafers are supported in a cassette and are withdrawn one at time and placed on a support. The wafer is then oriented in an implantation orientation so that the ion beam strikes the single wafer. These serial implanters use beam shaping electronics to deflect the beam from its initial trajectory and often are used in conjunction with coordinated wafer support movements to selectively dope or treat the entire wafer surface.
Ion sources that generate the ion beams used in existing implanters are typically so called arc ion sources and can include heated filament cathodes for creating ions that are shaped into an appropriate ion beam for wafer treatment. U.S. Pat. No. 5,497,006 to Sferlazzo et al concerns an ion source having a cathode supported by a base and positioned with respect to a gas confinement chamber for ejecting ionizing electrons into the gas confinement chamber. The cathode of the '006 patent is a tubular conductive body and endcap that partially extends into the gas confinement chamber. A filament is supported within the tubular body and emits electrons that heat the endcap through electron bombardment. The heated endcap then thermionically emits the ionizing electrons into the gas confinement chamber.
U.S. Pat. No. 5,763,890 to Cloutier et al also discloses an arc ion source for use in an ion implanter. The ion source includes a gas confinement chamber having conductive chamber walls that bound a gas ionization zone. The gas confinement chamber includes an exit opening to allow ions to exit the chamber. A base positions the gas confinement chamber relative to structure for forming an ion beam from ions exiting the gas confinement chamber.
Current materials defining the ion source chamber are typically formed of refractory metals and/or graphite, wherein the more commonly used refractory metals include tungsten, molybdenum, tantalum, and the like. Corrosion of these materials can occur when ionizing fluorine-based compounds such as BF3, GeF4, SiF4, B2F4 and/or oxygen-based compounds such as CO and CO2, which can dramatically shorten the lifetimes of these materials as well as deleteriously introduce impurities during ion implantation. For example, ionization of fluorine containing compounds can produce F−, which can react with exposed surfaces containing the currently employed refractory metal of tungsten, molybdenum, tantalum, graphite, and the like. For example, MoFx, WFx, TaFx, and the like can be formed upon exposure to F− ions, wherein x is an integer of 1 to 6 in most instances. These materials are corrosive by themselves and the presence of these corrosive materials within the chamber can further propagate a halogen cycle where the deposition of these materials within the ion source chamber may occur, which significantly shortens operating lifetimes of these components. When ionizing oxygen-based compounds such as CO and CO2, the formation of the corresponding refractory oxide can also cause erosion of the ion source components within the ion source chamber including but not limited to the cathode, liners, cathode shield (i.e., cathode repeller), repeller (i.e., anti-cathode), source aperture slit (i.e., ion source optics plate), and the like, thereby shortening operating lifetimes and requiring replacement.
In addition to the corrosive problems noted above that affect operating lifetimes, the refractory metal fluoride and/or refractory metal oxides that are generated upon exposure to ionized fluorine and/or oxygen species are oftentimes volatile by themselves. Unfortunately, these volatile compounds can be transported along with the desired dopant ions to the substrate being processed, which directly affects device performance and yield.
Therefore, a need exists to for improved ion source chamber components in ion implantation systems, wherein the improvement provides equivalent or better performance to the currently utilized materials such as, for example, the tungsten-, molybdenum-, and like refractory-based materials, yet are generally inert during processing so as to provide extended operating lifetimes without generating volatile compounds affecting device performance and yield.